Centrifugal compressor

ABSTRACT

A centrifugal compressor is equipped with a resin housing, that is made in a manner such that compression efficiency is not lost and merits such as weight and cost reduction are not cancelled out. The turbo charger housing comprises a volute section, a channel formation section, and an impeller that is fixed in the center of the channel formation section to a rotating shaft. An annular shroud is disposed in a recess formed on the inner wall of the channel formation section. The annular shroud forms the outer walls of a channel and a diffuser for compressed gas. The edge of the annular shroud spaced from a partition wall, which forms a portion of a bearing housing. If the resin housing undergoes thermal deformation, the dimensions of a gap between the annular shroud and the curved profile of a blade is maintained.

TECHNICAL FIELD

The present invention relates to a centrifugal compressor provided with a housing made of resin which is used in a turbocharger or the like.

BACKGROUND ART

A turbocharger installed in a vehicle or the like, drives a compressor by a exhaust turbine which is driven by energy of exhaust gas, compresses intake air in the compressor and supplies the compressed air to an engine. This type of turbocharger is provided with a bearing housing between a turbine housing of the exhaust turbine and a compressor housing for a turbo compressor. The bearing housing movably supports the rotating shaft rotatably. On the rotating shaft arranged through the bearing housing, an impeller of the turbo compressor and a wheel of the exhaust turbine are fixed.

The bearing housing houses a bearing mechanism for movably supporting the rotating shaft rotatably. Between the bearing housing and the compressor housing, a seal wall is interposed to form a flow path for gas which is enclosed in the compressor housing and is to be compressed.

A spiral-shaped housing such as a housing for a turbocharger, is normally made of cast aluminum, cast iron or the like. In recent years, for the purposes of making the housing lighter and at lower cost, a housing made of resin is used.

Disclosed in Patent Literature 1, is a housing for a turbo charger, which is made of thermoplastic resin. Disclosed in Patent Literature 2, is a housing for a centrifugal compressor, which has a double-wall structure made of thermoset resin and thermoplastic resin.

Patent Literature 3 discloses the invention to improve compressor efficiency and to prevent an impeller from being damaged. The inner wall of the compressor housing, which faces a flow path of the gas to be compressed, is made of resin material which is highly machinable. A clearance between the inner wall of the compressor housing and a curved profile of the impeller of the compressor is set small to improve compressor efficiency and to prevent the impeller from being damaged when the impeller comes in contact with the inner wall of the compressor housing.

In Patent Literature 4, an inner wall of a path forming section of a compressor housing of a turbocharger, is made of resin material which is highly machinable so as to reduce a cost thereof.

In Patent Literature 5, for the same reason as Patent Literature 3, an inner wall of a path forming section of a compressor housing of a turbocharger, is made of resin material.

FIG. 10 is a schematic view of a compressor of a turbo charger 100 provided with a resin housing. FIG. 10 shows an impeller 104 fixed to a rotating shaft 102 and a plurality of blades 106 extending radially from the impeller 104. An outer edge of the blade 106 forms a curved profile 106 a. A housing 110 is arranged around the blades 106. The housing 110 includes a volute section 110 a which forms a volute part s and a path forming section 110 b which forms a flow path c for the gas to be compressed.

The path forming section 110 b is arranged to surround the blades 106. The flow path c is formed by a hub face 108 of the impeller and an inner wall of the path forming section 110 b. The flow path c curves from an axial direction of the rotating shaft 102 (a direction of an arrow a) into a radial direction of the impeller 104 (a direction of an arrow b). A section extending from the flow path c where the blades 106 are arranged to the flow path d which is arranged on an outlet side of the flow path c, functions as a diffuser (static-pressure increasing region).

By rotation of the impeller 104, the gas to be compressed is drawn into the flow path c in the direction of the arrow a, and reaches the blades 106 to increase an absolute flow velocity of the gas. The intake gas whose velocity is increased in the flow path c, is directed into the direction of the arrow b to enter the diffuser d. As the intake gas advances in the diffuser d, the intake gas is compressed and then exhausted to the volute part s.

The compressor achieves high compression efficiency with a small clearance T between the inner wall of the path forming section 110 b and the curved profile 106 a of the impeller 104. The temperature of the gas to be compressed is raised by compressing the gas in the compressor. In the case of using the housing 110 made of resin, the resin has a higher thermal expansion rate than materials such as metal and thus, the housing 110 expands being exposed to the heat of the gas and thermally deforms in a direction of an arrow e as indicated by a dotted line 110′. By this, the clearance T becomes wider and the gas leaks from the wider clearance T, resulting in a decrease in compression efficiency.

In the resin housing, if the impeller 104 is damaged and broken pieces thereof fly in the housing, it is necessary to prevent the broken pieces from flying out of the housing. To take measures against the issue, it is possible to increase a thickness of the housing 110 or to provide a reinforcing rib 112 in the housing 110. However, such measures offset advantages of using the resin housing from the perspective of weight saving and cost reducing.

CITATION LIST Patent Literature [PTL 1]

-   DE 10260042 A1

[PTL 2]

-   EP 1830071 A2

[PTL 3]

-   JP 9-170442 A

[PTL4]

-   JP2001-234753 A

[PTL5]

-   JP2002-256878 A

SUMMARY OF INVENTION Technical Problem

It is an object of the present invention to provide a centrifugal compressor equipped with a resin housing, which can obtain advantages such as a weight saving and cost reduction without decreasing compression efficiency.

Solution to Problem

To achieve the above object, the present invention provides a centrifugal compressor which may include, but is not limited to: an impeller which includes a plurality of blades fixed radially to a rotating shaft; a resin housing which is arranged around the impeller and which is made of resin. Between the impeller and the resin housing, a flow path for a gas to be compressed may be formed by an outer periphery of the impeller and an inner wall of a path forming section of the resin housing, the gas flowing in the flow path from an axial direction to a radial direction of the impeller. The inner wall of the path forming section of the resin housing may be carved to form a depression in which an annular shroud is provided, the annular shroud being made of one of a metal and a ceramic material and forming an outer surface of the flow path where the impeller is arranged and an outer surface of a diffuser which is arranged on an outlet side of the flow path. The annular shroud may be fixed to a wall facing the annular shroud at the diffuser.

According to the present invention, the inner wall of the path forming section of the resin housing is carved to form the depression in which the annular shroud made of metal or ceramic material is provided. The annular shroud forms an outer surface of the flow path where the impeller is arranged and an outer surface of a diffuser which is arranged on an outlet side of the flow path. The annular shroud is made of a material which has a higher strength and a lower thermal expansion rate than resin (engineering plastic), such as aluminum and carbon steel, or ceramic. The annular shroud is fixed to the wall facing the annular shroud at the diffuser.

The annular shroud is made separately from the resin housing and is not joined to the inner wall of the resin housing. Thus, the thermal deformation of the resin housing does not affect the annular shroud, allowing the space between the annular shroud and the curved profile of the impeller to remain the same. Therefore, the compression efficiency is not affected. With use of the annular shroud, it is not necessary to make the resin housing thicker or to provide a reinforcement rib in the resin housing. As a result, it does not offset merits such as weight saving and cost reduction.

In the present invention, the centrifugal compressor may be provided with a seal ring housed in a groove which is disposed between a rear face of the annular shroud and the depression of the inner wall of the path forming section. By this, even when the resin housing thermally deforms, leaving a gap between the inner wall of the resin housing and the annular shroud, it is possible to prevent the gas from entering the gap and the gas on the downstream side of the impeller from flowing back to the inlet side of the impeller. As a result, the compression efficiency does not decrease.

At the inlet of the impeller, the temperature of the gas is low as well as the temperatures of the annular shroud and the inner wall of the path forming section of the resin housing. Thus, an inexpensive rubber O-ring can be used as the seal ring. Further, the diameter of the inlet of the impeller is small and thus, the O-ring of a small diameter can be used. Accordingly, it is possible to achieve the cost reduction of the seal ring.

In addition to the above structure, the path forming section of the resin housing may be separable at a separating plane into an upstream portion and a downstream portion in a direction of flow of the gas, the separating plane being positioned at the groove for housing the seal ring. By this, it is possible to eliminate a thick-walled portion of the resin housing. This prevents air bubbles or the like remaining in the resin housing during injection molding of the resin housing. As a result, the quality of the resin housing is improved and the yield is improved, thereby preventing cost increase.

Further, by placing the separating plane to the groove for housing the seal ring, there is no need for carving to form the groove and thus, the molding manufacturing of the resin housing becomes easy, resulting in reducing the processing cost.

In the present invention, the resin housing may have a slit in one of the path forming section and a separated part of the path forming section, the split being formed in an axial direction of the rotating shaft and opening to outside. By this, the thick-walled portion of the resin housing can be eliminated. This eliminates air bubbles or the like remaining in the resin housing during injection molding of the resin housing. As a result, the quality of the resin housing is improved and the yield is enhanced, thereby preventing cost increase.

In the present invention, between the annular shroud and the path forming section of the resin housing, a circulation space for the gas may be formed, and at least two communication openings may be formed along a flow direction of the gas such that the communication space and the flow path are in communication with each other through the communication openings to allow the gas to flow in the communication space. In this manner, when the flow rate of the gas is low, a portion of the gas enters the communication space through the opening on the downstream side and then returns to the flow path from the opening on the upstream side, thereby forming a circulation flow. As the circulation flow joins the flow of the gas at the inlet of the impeller, it is possible to keep the flow rate of the gas at the inlet of the impeller not less than a stall limit. As a result, it is possible to lower a lower limit of the flow rate of the compressor.

In contrast, when the flow rate of the gas is high, a portion of the gas enters the communication space from the opening on the upstream side and then returns to the flow path from the opening on the downstream side. Thus, the upper limit of the flow rate of the gas is enhanced. As a result, the upper limit and the lower limit of the flow rate of the gas, which is allowable for operation of the centrifugal compressor, can be expanded.

In the present invention, the path forming section of the resin housing may be reinforced by an annular reinforcement layer covering an outer wall of the path forming section, the reinforcement layer being made of glass fiber. By providing on the outer wall of the path forming section the reinforcement layer made of glass fiber with high tension strength, when the impeller is damaged, the broken pieces of the impeller or the broken impeller does not penetrate through the wall of the path forming section and thus, the wall thickness of the path forming section, reinforced by the reinforcement layer, can be thinner. As a result, it is possible to reduce the production cost of the resin housing.

Advantageous Effects of Invention

According to the centrifugal compressor of the present invention, the centrifugal compressor is provided with: an impeller which includes a plurality of blades fixed radially to a rotating shaft; a resin housing which is arranged around the impeller and which is made of resin. Between the impeller and the resin housing, a flow path for a gas to be compressed is formed by an outer periphery of the impeller and an inner wall of a path forming section of the resin housing, the gas flowing in the flow path from an axial direction to a radial direction of the impeller. The inner wall of the path forming section of the resin housing is carved to form a depression in which an annular shroud is provided, the annular shroud being made of one of a metal and a ceramic material and forming an outer surface of the flow path where the impeller is arranged and an outer surface of a diffuser which is arranged on an outlet side of the flow path. The annular shroud is fixed to a wall facing the annular shroud at the diffuser.

Therefore, the clearance between the annular shroud and the curved profile of the blade can be kept at a set dimension, thereby maintaining a high compression efficiency, and by providing the annular shroud, there is no need to make the resin housing thicker or to provide a reinforcement rib in the resin housing and thus, the benefits such as a lighter weight or cost reduction of the resin housing can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a centrifugal compressor in relation to a first embodiment, to which the present invention is applied.

FIG. 2 is a side view of a centrifugal compressor in relation to a second embodiment, to which the present invention is applied

FIG. 3 is a side view of a centrifugal compressor in relation to a third embodiment, to which the present invention is applied.

FIG. 4 is a side view of a centrifugal compressor in relation to a fourth embodiment, to which the present invention is applied.

FIG. 5 is a side view of a centrifugal compressor in relation to a fifth embodiment, to which the present invention is applied.

FIG. 6 is a side view showing a modified example of the fifth embodiment.

FIG. 7 is a side view of a centrifugal compressor in relation to a sixth embodiment, to which the present invention is applied.

FIG. 8 is a side view showing a modified example of the sixth embodiment.

FIG. 9 is a side view of a centrifugal compressor in relation to a seventh embodiment, to which the present invention is applied.

FIG. 10 is a side view showing a part of a conventional centrifugal compressor.

FIG. 11 is a side view of a centrifugal compressor showing a comparison between a thick resin housing and a reinforcement rib.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shape, its relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present invention.

First Embodiment

A first embodiment of a centrifugal compressor to which the present invention is applied is explained in reference to FIG. 1. In FIG. 1, a rotating shaft 12 is arranged in a center of a resin housing 20, and an impeller 14 having a plurality of blades 16 fixed radially is fixed to an outer periphery of the rotating shaft 12. A hub face 18 of the impeller 14 curves from an inlet side to an outlet side in a flow direction of a gas to be compressed (a direction indicated with an arrow a) from an axial direction to a radial direction of the rotating shaft 12. The resin housing 20 includes a volute section 20 a which forms a volute part s and a path forming section 20 b which forms a flow path c for the gas to be compressed.

The outer edge of the blade forms a curved profile 16 a. Along the curved profile 16 a, an inner wall 20 b ₁ of the path forming section 20 b of the resin housing 20 is formed. The inner wall 20 b ₁ is carved to form a depression 20 b ₂ in which an annular shroud is inserted. The rotating shaft 12 is rotatably and movably supported by a bearing provided in a bearing housing 30. The bearing is not shown in the drawing. The spiral-shaped volute section 20 a forms a volute part s. A wall 32 constituting a part of the bearing housing 30 is arranged facing the volute part s formed by the spiral-shaped volute section 20 a. An end of the volute section 20 a is fixed to the wall 32 via a seal ring 34.

In the depression 20 b ₂ formed in the inner wall 20 b ₁ of the path forming section 20 b, an annular shroud 22 is inserted. A downstream end of the annular shroud 22 which is on a downstream side in a direction of the gas flow, projects into the volute part s. The downstream end of the annular shroud 22 is fixed to the wall 32 by a bolt 26 via a spacer 24. The annular shroud 22 is made of a metal which has a higher strength and a lower thermal expansion rate than resin, such as aluminum and carbon steel, or ceramic. The annular shroud 22 and the resin housing 20 are not connected. The clearance between the annular shroud 22 and the curved profile 16 a of the blade 16 is set as small as possible to maintain decent compression efficiency.

In this manner, the annular shroud 22 forms an outer surface of the flow path c and an outer surface of the diffuser d. The flow path c directs the gas from an axial direction of the impeller 14 (the direction of the arrow a) into a radial direction of the impeller (a direction of an arrow b). The diffuser d is arranged on an outlet side of the flow path c to convert the kinetic energy of the gas into a static pressure. An inner surface of the flow path c is formed by the hub face 18 of the impeller 14, whereas an inner surface of the diffuser d is formed by the wall 32.

With the above structure, the rotation of the impeller 14 causes the gas to be drawn in from an inlet side of the blade 16, through the flow path c and the diffuser d and then converted into the static pressure. The compressed gas flows through the diffuser d and then enters the volute part s. A plurality of bolts 26 are arranged at intervals in the circumferential direction of the rotating shaft 12. However, the bolts 26 do not interfere with the flow of the gas.

According to the first embodiment, the annular shroud 22 is fixed to the wall 32 of the bearing housing 30 solely by the bolts 26 and the annular shroud 22 and the resin housing 20 are separated from each other. Thus, heat deformation of the resin housing 20 does not reach the annular shroud 22. Therefore, even when the resin housing 20 thermally deforms, the clearance between the annular shroud 22 and the blade 16 does not change. As a result, the compression efficiency is not affected.

By providing the annular shroud 22, it is no longer necessary to make the resin housing 20 thicker or to provide a reinforcement rib or the like in the resin housing 20. As a result, it does not offset merits such as a lighter weight and cost reduction.

Second Embodiment

A second embodiment of the centrifugal compressor to which the present invention is applied is explained in reference to FIG. 2. In the second embodiment, as shown in FIG. 2, the downstream end 22 a of the annular shroud 22 has two bending sections. The annular shroud 22 bends at a first bending section to extend to a position where the downstream end comes in contact with the wall 32 of the bearing housing 30 and bends at a second bending section to form a flange part 22 a ₁. The wall 32 is formed with a depression 32 a in which the flange part 22 a ₁ is fitted. The flange part 22 a ₁ is connected to the depression 32 a by a bolt 40.

The flange part 22 a ₁ is formed partially around the impeller 14. Thus, the downstream end 22 a of the shroud 22 does not block the diffuser d or interfere with the flow of the gas in the diffuser. The rest of the structure is substantially the same as that of the first embodiment. The same parts are indicated by the same reference numerals.

According to the second embodiment, the annular shroud 22 is a single-piece member which extends to the flange part 22 a ₁ and is processed by pressure forming. Therefore, in addition to the function effects achieved in the first embodiment, the annular shroud 22 is easy to manufacture by pressure-forming and it is easy to connect the annular shroud 22 to the wall 32 as there is no need for the spacer 24 which is provided in the case of the first embodiment, in order to connect the annular shroud 22 to the wall 32.

Third Embodiment

Next, a third embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to FIG. 3. In the third embodiment, the depression 20 b ₂ carved in the inner wall 20 b ₁ of the path forming section 20 b of the resin housing 20, is carved to form a groove 52. In the groove 52, a rubber or resin seal ring 50 is inserted. The rest of the structure is substantially the same as that of the second embodiment.

According to the third embodiment, even when the resin housing 20 thermally deforms, creating a gap between the depression 20 b ₂ and a rear face of the annular shroud 22, it is possible to prevent the entry of the gas between the depression 20 b ₂ and the annular shroud 22 by means of the seal ring 50. Therefore, the compression efficiency of the compressor does not decrease.

At an inlet of the impeller 14, the temperature of the gas is approximately the same as the ambient air and is low. At the inlet of the impeller, the temperatures at the annular shroud 22 and the path forming section 20 b are also low and thus, an inexpensive rubber O-ring can be used. Further, at the inlet of the impeller 14, the gas path in which the gas flows does not curve in the radial direction and the diameter of the impeller 14 is small and thus, the O-ring of a small diameter can be used. Therefore, it is possible to achieve the cost reduction of the seal ring 50.

Fourth Embodiment

Next, a fourth embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to FIG. 4. FIG. 4 shows the resin housing 20 of the fourth embodiment. The path forming section 20 b of the resin housing 20 is separable into two resin separated portions 60 and 62. The path forming section 20 b is separated into the separated portions 60 and 62 at a separating plane 64 having a stepped portion 64 a in a center of a thick-walled portion thereof. The separating plane 64 is disposed such that one end there of is positioned at the groove where the seal ring 50 is housed. The rest of the structure is substantially the same as that of the third embodiment.

According to the fourth embodiment, the path forming section 20 b is separated into the separated portions 60 and 62 and thus, the thickness of the path forming section 20 b is reduced. This prevents, during molding manufacturing of the resin housing 20, air bubbles from remaining in the path forming section 20 b. Therefore, the quality of the resin housing 20 is improved and with improved yield rate, the production cost of the resin housing is reduced.

Further, by placing the separating plane 64 to the groove 52, there is no need for carving the path forming section 20 b to form the groove 52, and a corner of the separated portion 60 is processed and thus, the molding manufacturing of the separated portion 60 becomes easy, resulting in reducing the processing cost.

Fifth Embodiment

Next, a fifth embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to FIG. 5. In the fifth embodiment, the thick-walled portion of the path forming section 20 b of the resin housing 20, has a slit V in a direction perpendicular to the thickness of the path forming section, i.e. in a direction approximately parallel to the rotational axis of the impeller 14. The slit V opens to outside. The rest of the structure is substantially the same as that of the third embodiment.

According to the fifth embodiment, by forming the slit V in the path forming section 20 b, the thick-walled portion of the path forming section 20 b can be eliminated. This eliminates air bubbles or the like remaining in the resin housing during injection molding of the resin housing 20. As a result, the quality of the resin housing 20 is improved and the yield is improved, thereby preventing cost increase.

A modified example of the fifth preferred embodiment is explained in reference to FIG. 6. According to the modified example shown in FIG. 6, the separated portion 60 of the separated portions 60 and 62 of the resin housing 20, is formed with a slit Va inside the thick-walled portion of the path forming section 20 b. The slit Va is formed in the direction perpendicular to the thickness of the path forming section, i.e. in the direction approximately parallel to the rotational axis of the impeller 14. And, an end of the slit Va opens to outside. The rest of the structure is substantially the same as that of the fourth embodiment.

According to the modified example, in addition to the function effects achieved in the fourth embodiment, by providing the slit Va in the path forming section 20 b of the separated portion 60, the thick-walled portion of the separated portion 60 is eliminated. This eliminates air bubbles or the like remaining in the resin housing during injection molding of the separated portion 60 of the resin housing 20. As a result, the quality of the separated portion 60 is improved and the yield is improved, thereby preventing cost increase.

Sixth Embodiment

A sixth embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to FIG. 7. In the sixth embodiment, the inner wall 20 b ₁ of the path forming section 20 b is carved to form the depression 20 b ₂ in the flow path c and the depression 20 b ₂ is further carved to form a circulation space 70 on a rear side of the annular shroud 22. The annular shroud 22 includes an upstream through-hole 72 and a downstream through-hole 74 where the annular shroud faces the circulation space 70. The rest of the structure is substantially the same as that of the second embodiment.

According to the sixth embodiment, when the flow rate of the gas is low, a portion of the gas flowing in the flow path c enters the circulation space 70 from the downstream through-hole 74 and then returns to the flow path c from the upstream through-hole 72, thereby forming a circulation flow. As the circulation flow joins the flow of the gas at the inlet of the impeller 14, thereby preventing the impeller 14 from stoll. As a result, it is possible to lower a lower limit of the flow rate of the compressor.

In contrast, when the flow rate of the gas is high, a portion of the gas flowing in the flow path c enters the circulation space 70 from the upstream through-hole 72 and then returns to the flow path c from the downstream through-hole 74. By this, it is possible to increase an upper limit of the flow rate of the gas. As a result, the range of the flow rate of the gas, which is allowable for operation of the centrifugal compressor, can be expanded.

Next, a modified example of the sixth embodiment is explained in reference to FIG. 8. In a manner similar to the sixth embodiment, the circulation space 70 is formed on the rear side of the annular shroud 22. At the entrance to the flow path c, a circular slit opening 76 is formed between an end of the annular shroud 22 which is on an inlet side and the inner wall 20 b ₁ of the path forming section 20 b. On a downstream side of the slit opening 76, the through-hole 74 which is the same as the through-hole 74 of the sixth embodiment, is formed in the annular shroud 22 path c. The rest of the structure is substantially the same as that of the sixth embodiment.

In comparison with the sixth embodiment, this modified example has an advantage of easier manufacturing of the annular shroud 22 and the path forming section 20 b as the number of through-holes formed in the annular shroud 22 is reduced and there is no need to carve the inner wall surface 20 a of the path forming section 20 b to form the depression 20 b ₂ in which the annular shroud 22 is fit. Further, the annular slit opening 76 is formed and thus, the opening area of the slit opening can be increased and it is easy to form the slit opening.

Seventh Embodiment

A seventh embodiment in which the present invention is applied to a centrifugal compressor is explained in reference to FIG. 9. In the seventh embodiment, the path forming section 20 b of the resin housing 20 is made thinner, and behind the path forming section 20 b, an annular glass fiberboard 80 is provided. The rest of the structure is substantially the same as that of the second embodiment.

When the impeller 14 is damaged, the path forming section 20 b must have enough strength to prevent the broken pieces of the impeller or the broken impeller 14 from breaking through the resin housing 20. According to the seventh embodiment, the glass fiberboard 80 is provided on the rear side of the path forming section 20 b for reinforcement and thus, the path forming section 20 b can be thin. With reduced thickness of the path forming section 20 b, it is possible to eliminate air bubbles or the like remaining in the resin housing 20 during injection molding of the resin housing 20. As a result, the quality of the resin housing 20 is improved, thereby improving the yield during the molding manufacturing of the resin housing and preventing cost increase.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible, in a centrifugal compressor which can be used in a turbocharger or the like, to achieve a lighter weight and cost reduction of the housing without a decrease in compression efficiency even when the housing is thermally deformed. 

1. A centrifugal compressor comprising: an impeller which comprises a plurality of blades fixed radially to a rotating shaft; and a resin housing which is arranged around the impeller and which is made of resin, wherein between the impeller and the resin housing, a flow path for a gas to be compressed is formed by an outer periphery of the impeller and an inner wall of a path forming section of the resin housing, the gas flowing in the flow path from an axial direction to a radial direction of the impeller, wherein the inner wall of the path forming section of the resin housing is carved to form a depression in which an annular shroud is provided, the annular shroud being made of one of a metal and a ceramic material and forming an outer surface of the flow path where the impeller is arranged and an outer surface of a diffuser which is arranged on an outlet side of the flow path, and wherein the annular shroud is fixed to a wall facing the annular shroud at the diffuser.
 2. The centrifugal compressor according to claim 1, further comprising: a seal ring housed in a groove which is disposed between a rear face of the annular shroud and the depression of the inner wall of the path forming section.
 3. The centrifugal compressor according to claim 2, wherein the path forming section of the resin housing is separable at a separating plane into an upstream portion and a downstream portion in a direction of flow of the gas, the separating plane being positioned at the groove for housing the seal ring.
 4. The centrifugal compressor according to claim 1, wherein the resin housing has a slit in one of the path forming section and a separated part of the path forming section, the split being formed in an axial direction of the rotating shaft and opening to outside.
 5. The centrifugal compressor according to claim 1, wherein between the annular shroud and the path forming section of the resin housing, a circulation space for the gas is formed, and at least two communication openings are formed along a flow direction of the gas such that the communication space and the flow path are in communication with each other through the communication openings to allow the gas to flow in the communication space.
 6. The centrifugal compressor according to claim 1, wherein the path forming section of the resin housing is reinforced by an annular reinforcement layer covering an outer wall of the path forming section, the reinforcement layer being made of glass fiber. 